A. Field of the Invention
This invention relates to the replenishing of substance within liquid storage tanks and, in particular, replenishing such tanks when the substance in the tanks produces erratic pressure readings when drawn or replenished.
B. Background Art
Many liquid products are delivered in bulk to a user's site where they are held in a storage tank, from which either intermittent or continuous withdrawal may occur. Examples include liquid fuels such as domestic heating oil, liquid chemicals employed in manufacturing processes, and liquified gases such as oxygen or nitrogen.
For some applications, it is important that the tank never run dry. For example, liquified nitrogen is a source of inert nitrogen gas for submerging various industrial processes and operations. If the supply of nitrogen runs out during a process run, an entire production lot may be destroyed. Nitrogen gas is also used to purge storage tanks from which flammable liquids are being withdrawn to avoid the entrance of air and the subsequent formation of explosive mixtures. Exhaustion of nitrogen during this process can result in an explosion.
Two methods were commonly employed to insure that liquid was available at all times in these storage tanks. The first method was to anticipate from past experience the rate of withdrawal of liquid from the tank, and to extrapolate to estimate the time at which replenishment was needed. This approach assumed that current usage patterns were similar to prior usage patterns. If depletion were more rapid than anticipated because of increased use or leakage, the tank could empty before the normal replenishment time was reached. Likewise, if usage were unexpectedly curtailed, an unnecessary trip by the supplier's delivery vehicle resulted in the addition of only a small amount to top off the tank.
The second approach was for the user to periodically measure the amount of substance remaining in the tank, and notify the supplier when a delivery was required. This method depended entirely upon the diligence of the user in adhering to a schedule of tank readings and the user's ability to recognize the necessity of a request for a delivery.
In some applications usage tends to be more concentrated in certain periods of the day than others. For example there may be an application where there is very heavy usage between noon and four o'clock in the afternoon. Thus reaching a predetermined low level in this situation at eleven o'clock in the morning could be more critical than reaching such a level at five o'clock in the afternoon. Therefore it is useful to know the usage pattern when interpreting a tank reading in order to determine the urgency for refilling a tank.
Furthermore, it was not possible, in the case of cryogenic liquids, for the user to obtain a direct measurement of the substance level. Typically, the user had available only pressure readings from the tank from which to determine the substance level. These pressure readings included (1) the pressure in the vapor space at the tank and (2) the differential pressure which is the difference between the vapor space pressure and the pressure at the bottom of the tank, called the substance weight pressure.
The relationship between these two variables, the vapor space pressure and the differential pressure, and the actual substance volume is a complex high order polynomial. Solution charts for this polynomial have been prpared. Thus, to get an accurate measurement of the substance volume, the user was required to locate the correct chart entry depending on the pressure readings. These charts were different for different substances and for different tanks.
Further complicating these problems was the fact that, when substance was supplied to a cryogenic tank the vapor space pressure readings became erratic. This happens when the pump forces new substance into the tank when the substance is at a different temperature than that of the tank. The temperature differential results in vaporization of liquid which in turn causes turbulence in the tank and unpredictable fluctuations. Thus, for practical purposes it is extremely difficult to determine substance level under conditions of replenishing.
For purposes of considering the patentability of the invention disclosed and claimed, a brief patentability search was conducted with respect to parent application Ser. No. 664,817. The patents identified to be of possible interest in that search were:
______________________________________ Patent No. Inventor ______________________________________ Re. 19,868 Schontzler et al. 4,201,240 Case 4,250,750 Martinec et al. 4,252,097 Hartford et al. 4,296,472 Sarkis 4,313,114 Lee et al. 4,353,245 Nicolai 4,361,037 Hauschild et al. 4,402,048 Tsuchida et al. 4,434,657 Matsumura et al. 4,437,162 Kato 4,441,157 Gerchman et al. ______________________________________